Facilities for long-term storage of cryogenic liquids in space are required for many missions, in particular, those that make use of stored cryogens such as liquid hydrogen for purposes of cooling of infrared detectors and other equipment that has an extremely low operating temperature. For operations in a microgravity environment of space, cryogenic storage systems preferably make use a phase separator to prevent loss of the cryogenic liquid and to take advantage of the latent heat of vaporization of the liquid cryogen before it is vented from the system.
One approach proposed for phase separators in space cryogenic systems is a thermodynamic vent system in which both liquid and vapor phases are discharged through a Joule-Thompson orifice. After leaving this orifice, the two-phase discharge is forced to flow through a heat exchanger tube that is coupled to the tank wall and/or to heat shields around the dewar. Ideally, the liquid phase discharge should all be converted to vapor in the heat exchanger tube to make use of the latent heat of evaporation of the discharged liquid. This approach presents disadvantages in that two-phase flow in zero gravity is poorly understood, with little existing experimental data. Thus, a good deal of uncertainty exists in the assumption that all of the liquid discharged from the tank will be converted to vapor before release from the system. Also, basic thermodynamics show that more efficient cooling is achieved if all liquid-vapor conversion takes place in the tank and no liquid at all is discharged from the tank.
U.S. Pat. No. 4,412,851, issued Nov. 1, 1983, shows a phase separator for use in zero gravity that has an inlet communicating with a cryogen reservoir and an outlet to space with a transfer chamber in between, with flow restrictors at the ends of the chamber and a pair of obturators between the flow restrictors, the obturators being operated alternately by a control system.
Phase separators using a porous plug of sintered high-thermal conductivity metal have also been used in cryogenic systems, the porous plug providing for phase separation by capillary action. Porous plugs as fabricated by prior methods and used or proposed for use in space cryogen systems present a significant disadvantage in that their porous structure is complex and irregular, and the individual pores do not extend entirely through the plug but rather form complex interconnected short flow paths. Such a structure does not lend itself to an analysis so as to enable design of an efficient operating system. A plug having holes of uniform diameter penetrating all the way through and uniformly spaced apart is needed to enable development of analytical models useful for design of an actual phase separator. Provision of a trapping plug with holes having these characteristics has not been possible previously for plugs needed for certain space cryogen systems owing to the extremely small diameters required for the holes, in particular, diameters in the low micron to submicron size. No practical method has been available for fabrication of plugs with uniform holes of such small sizes.
Trapping plugs for zero g phase separators should have the capability for operating under either wet or dry conditions without loss of liquid trapping, and they should be amenable to analytical treatment that enables prediction of thermodynamic operation for both conditions. The plugs should also be suitable for installation into the walls of typical cryogen storage dewars.